1. Field of the Invention
The present invention relates to a common mode filter that suppresses common mode noise that propagates through a differential transmission line, and to an ESD-protection-circuit-equipped common mode filter.
2. Description of the Related Art
Conventionally, a common-mode choke coil is used to suppress common mode noise attempting to propagate through a differential transmission line. A common-mode choke coil includes two coils, the two coils being configured such that magnetic fields generated around the coils by differential mode signals (normal mode signals) cancel each other out and the magnetic fields strengthen each other for common mode signals (common mode noise), as described in Japanese Unexamined Patent Application Publication No. 2003-133135, for example.
Generally, it is necessary that the coils each have a large inductance and that there be a large coupling coefficient between the coils in such a common-mode choke coil.
However, when the inductances of the coils are large and the coupling coefficient between the coils is large, the self-resonant frequency is low and the transmission loss of a signal is large. Consequently, it is difficult to use such a common-mode choke coil in a differential transmission line that is used in a high frequency band and that is required to have low transmission loss such as a differential transmission line for a high-speed interface like USB or HDMI (registered trademark), for example.